<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Resizable iframes</title>
  <link rel="stylesheet" href="https://code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
  <style>
      #container {
          display: flex;
          height: 100vh;
      }
      #left, #right {
          width: 50%;
          float: left;
          overflow: hidden;
      }
      .ui-resizable-e{
          width: 40px;
          background: red;
      }
  </style>
</head>
<body>

<div id="container">
  <div id="left">
    <iframe src="sftp.html" style="width: calc(100% - 40px)"></iframe>
  </div>
  <div id="right">
    <iframe src="ssh.html"></iframe>
  </div>
</div>

<script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
<script src="https://code.jquery.com/ui/1.12.1/jquery-ui.min.js"></script>
<script>
  $(function() {
    $("#left").resizable({
      handles: "e",
      resize: function(event, ui) {
        var leftWidth = ui.size.width;
        $("#right").width($("#container").width() - leftWidth);
      }
    });
  });
</script>

</body>
</html>